An automotive fuel tank is normally provided with a breather tube for the purpose of allowing air and/or fuel vapor to escape from the fuel tank. One end of the breather tube is located in a relatively high position inside the fuel tank while the other end is communicated with a canister device for absorbing fuel vapor and diverting it to the intake system of the internal combustion engine by suitable means. A relief valve is provided in the breather tube which allows the gas vapor in the fuel tank to escape into the canister device only when the pressure inside the fuel tank has risen above the atmospheric pressure by a certain degree.
When the fuel tank is being filled up with fuel, the vapor of the fuel is trapped in an upper part of the fuel tank in which the associated end of the breather tube opens into. When the fuel tank is substantially completely filled up, and the associated end of the breather tube is closed by the surface of the fuel, the internal pressure of the fuel tank above the surface level of the fuel starts rising sharply, and no further filling of fuel into the tank is possible because the fuel surface level of the fuel filler tube starts rising sharply, and this rise in the fuel surface level in the filler tube is detected by a sensor provided in the fuel pump nozzle inserted in the inlet end of the filler tube, thereby stopping the filling of fuel from the fuel pump.
Because the canister device is only capable of absorbing a limited amount of fuel vapor and is not capable of absorbing fuel in liquid form, the set pressure of the valve must be high enough to prevent the fuel in the fuel tank from reaching the canister device. As a result, the set pressure of the relief valve is such that the internal pressure of the fuel tank may reach a level which is substantially higher than the atmospheric pressure. Therefore, when the fuel cap is opened, typically for filling fuel into the fuel tank, the vapor in the fuel tank often placed under a substantial pressure tends to be released to the atmosphere by a substantial amount, and this is not desirable for the environment.
Therefore, it is conceivable to control the internal pressure of the fuel tank to be no higher than the atmospheric pressure by eliminating the valve in the passage leading to the canister device. However, this will create the problem that some of the fuel may flow into the canister device when filling the fuel tank with fuel.